One of the main reasons of cracking in semiconductor dies is imbalance of mechanical stress applied to the die backside by the push-up or ejector needles during pickup.
FIG. 1 is a schematic 100 showing a semiconductor die 102 being pushed by ejector needles 104. Force is applied by the ejector needles 104 on the backside of the semiconductor die 104. The uneven stress (caused by the ejector needles) may result in cracks 106 appearing on or within the semiconductor die 102.